Because of the innovations, it really has to be 1990-1995. It covered so much ground and evolution from what were basically street bikes with bells and balls (yes, those) on, to genuine mountain bikes that would go almost anywhere. Also would cover the introduction of proper suspension which was pivotal.
Best bike? So many to choose from I think it would be impossible to say. Best frame? Probably the Merlin USA handbuilt Ti frames. They were and remain so iconic that I think you'd find it hard to identify something objectively better. The nice thing about Merlin was they stood alone, over most team rivalries etc. Merlin were a small fish in a big pond, so quality mattered above all and it showed and continues to, decades later. Many have tried to do the same, but all had some kind of caveat.
Mainstream wise, what sticks in my mind from that era would be the Marin Team Ti, Kona Hei Hei, GT Xixang, Merlin Ti, Alpinestars Ti-Mega and maybe one or two others I can't recall just now.
But yes, for me, 1990-1995.